Difference in anode-cathode pair and the electrolyte used in making of Nickel Cadmium rechargeable batteries produce 1.2 V emf rather than 1.5 V produced by carbon-zinc or alkaline batteries.
The electric cell voltage is characteristic of the anode, cathode and the electrolyte.
We visualize the cell potential as a measure of the potential difference between two half cells in an electrochemical cell. This potential difference is caused by the ability of electrons to flow from one half cell to the other half.
The cell potential or cell's electromotive force, in short emf , depends on the contribution from the anode, called its oxidation potential.
And the cathode contributes its reduction potential.
The cell potential, therefore, can be written as
Standard Electrode Potentials in Aqueous Solution at
One of the limitations of NiCd rechargeable cells is Memory effect.
If the cell/battery is recharged before it is fully discharged, it remembers the partially discharged state.
For future charge - recharge cycles the battery behaves as if this partially discharged position is actually completely discharged state. This leads to loss of its capacity.
It is understood that it occurs due to the formation of Cadmium crystals inside the battery. These crystals are hard to dissolve in the electrolyte and are the ones responsible for the observed memory effect.
In order to overcome this effect the battery must not be charged unless the device, it is driving, indicates that battery has reached its fully discharged condition.